This relates generally to communications links, and more particularly, to high-speed input-output (I/O) communications links.
A typical communications link includes a transmitter, a receiver, and a channel that connects the transmitter to the receiver. The transmitter in one integrated circuit transmits a serial data bit stream to the receiver in another integrated circuit via the channel. Typical high-speed transmit data rates are 1 Gbps (gigabits per second) to 10 Gbps. Communications links operating at such high data rates are often referred to as high-speed serial links or high-speed input-output links.
In general, it is desirable to match the impedance of the transceiver (i.e., the transmitter and the receiver) with the impedance of one or more off-chip transmission lines serving as the channel to optimize signal integrity. In practice, however, the characteristic impedance of the transmission lines such as metal traces on a printed circuit board (PCB) is subject to variations due to PCB manufacturing defects (i.e., PCB traces often exhibit impedances that deviate from target impedance levels), PCB characteristic degradation due to semiconductor aging effects, environment effects from changes in temperature and humidity, and other sources of random/systematic variations.
In an effort to compensate for possible impedance mismatch with external traces, conventional integrated circuits are typically provided with an on-chip termination circuit at the transceiver. The on-chip termination circuit is configured to provide a fixed set of impedance values that is capable of supporting high speed communications using common industry communications standards. For example, the on-chip termination circuit can be configured to provide an impedance selected from one of 85 Ω, 100 Ω, 120Ω, and 150Ω. During device startup, one of these on-chip termination settings that provides the minimum amount of impedance mismatch may be selected. However, since the transmission line impedance in reality varies over time, it is challenging to achieve perfect impedance match using a fixed on-chip termination value.